Phase sensitive (PS) inversion recovery (IR) has previously been recognized as a method to obtain T1-weighted tissue differentiation with high contrast and relative insensitivity to inversion times (TI). T1-weighted imaging can be achieved by applying IR preparation and waiting a time called inversion time (TI), during which longitudinal magnetization recovers due to T1 relaxation, before acquiring imaging data. These methods require generation of a background phase reference.
Prior strategies for obtaining a phase reference have extracted this information from the image itself or have acquired a separate phase reference image without IR preparation. The first applied single-point Dixon technique (i.e. MRI method used for fat suppression and/or fat quantification) and chemical-shift information from a single echo by calculating phase gradients from neighboring pixels without acquiring phase reference data. The latter achieved PS IR reconstruction by acquiring phase reference data in a separate scan, which was demonstrated in brain imaging.
PS IR reconstruction with cardiac gating was proposed for late Gadolinium enhancement (LGE) cardiac MR scans as it is less sensitive to changes in contrast media concentration, especially if the selected TI time is shorter than the optimal TI (null point of healthy myocardial tissue). The system 100 of FIG. 1 (PRIOR ART) demonstrates the previously existing PS LGE image acquisition. Segmented LGE k-space and segmented phase reference k-space are acquired in multiple R-R intervals (i.e., “R-R interval” refers to the duration between subsequent R peaks in an echocardiogram (ECG) waveform, in other words, a heartbeat). However, this PS approach significantly prolongs the total scan time as it requires the use of a 2 R-R interval scan that acquires the primary LGE signal in IR-prepared R-R intervals 109 and phase reference data in non-IR-prepared R-R intervals 111. The phase reference scan is acquired in the non-IR-prepared R-R intervals 111 as it provides sufficient delay after the IR radiofrequency (RF) pulse to allow longitudinal magnetization (Mz) of tissue types to fully recover above the zero line (Mz=0). The phase reference scan is gated to the same cardiac phase as the primary LGE image using a fixed trigger delay (TD) to prevent motion mis-registration. The IR pulse prepared data 108 and phase reference data 110 are interleaved every other R-R interval. The IR prepared image 104 and phase reference image 106 are used to reconstruct a phase sensitive image 102.
The 2 R-R interval prolongs the overall scan time but also yields improved image signal-to-noise ratio (SNR) as the longitudinal magnetization is allowed to more fully recover to the equilibrium value prior to the next IR pulse and data readout. A single R-R interval scan has been proposed for 3D LGE and is used for magnitude (non-PS) image reconstruction. The single R-R scan has the advantage of halved scan time compared to a 2 R-R scan. When used in a 3D volume LGE acquisition, sufficient image SNR from a volume acquisition provides a minimally acceptable image SNR as a trade-off to a much shorter scan time. However, in order to utilize a PS reconstruction, a phase reference measurement needs to be obtained. This phase reference measurement requires that the longitudinal magnetization recover above the zero (Mz=0) line, necessitating a longer, 2 R-R interval.
The present method will address the issues of prolonged scan time, SNR, and longer 2 R-R intervals. The method will desirably allow a more time-efficient acquisition of LGE data that permits the acquisition of the primary normal myocardium-nulled LGE data as well as a phase reference measurement to allow the reconstruction of phase-sensitive inversion recovery images but at a significant reduction in scan time. Furthermore, the improved system will provide a solution that reduces scan time without sacrificing quality of the scan.